1. Field of the Invention
The present invention relates to an optical head which is capable of both forming a laser spot at the magnetic layer of a magneto-optical disk and supplying an external magnetic field to the magnetic layer of the magneto-optical disk. The present invention also relates to a coil assembly used for such an optical head.
2. Description of the Related Art
Conventionally, various kinds of magneto-optical disk apparatus have been used for writing and reading data in and from a magneto-optical disk. Taking a data-reading method by MSR (magnetically induced super resolution) for example, use may be made of a magneto-optical disk which includes laminated magnetic layers whose magnetic characteristics may vary depending upon temperature. Data stored in such a disk is read out from a region, within a laser spot, which is heated up to a particular temperature by a laser beam. For reading out data from the disk, a magnetic field needs to be supplied to an area corresponding to the laser spot.
To write desired data in the magneto-optical disk, the recording layer of the disk will be irradiated by a laser beam, while being supplied with an external magnetic field. When a region of the recording layer is heated up to or above the Curie temperature by the laser beam, the direction of magnetization in the region may be reversed, meaning that data is stored. For reversing the direction of magnetization, use may be made of e.g. a light modulation method or a magnetic field modulation method. In either way, it is necessary to provide appropriate means for generating an external magnetic field for performing data-recording.
As understood from the above, an apparatus used for writing data in a magneto-optical disk and reading out data therefrom needs to be provided with at least two functions; a first function for forming a laser spot on the surface of the disk, and a second function for generating an external magnetic field to perform the recording or reading of data. For the first function, an optical head is used, while for the second function, a magnetic head is used.
An optical head and a magnetic head may be located separately. Specifically, the former may be arranged in facing relation to the recording surface of the disk (i.e., on the side of the transparent substrate), whereas the latter may be arranged in facing relation to the opposite (back) surface of the disk.
Alternatively, an optical head and a magnetic head may be integrated into a single unit. An example of such a unit is disclosed in JP-A-2(1990)-18720. Referring to FIGS. 17-20 of the present application, the conventional unit (optical head) disclosed in this Japanese document will now be described below.
As shown in FIG. 17, the conventional apparatus includes a spindle (Sp) for rotating a magneto-optical disk (D) attached thereto, an arm (A) movable in radial directions of the disk (D), a mirror (M) mounted on an end of the arm (A), and a slider (S) supported by the arm (A) via a suspension arm (Sa). The slider (S) is provided with an objective lens (L), as shown in FIG. 18, and a coil block (Cb) disposed immediately under the objective lens (L).
As shown in FIG. 18, the coil block (Cb) includes a supporting base (B) and a coil (C) formed on the bottom surface of the supporting base (B). The supporting base (B), which may be made of silicon, is formed with a through hole (a) which resembles a reversed truncated pyramid. The through hole (a) is provided for allowing the passage of a laser beam converged by the objective lens (L). After passing through the hole (a), the converged laser beam reaches the disk (D) and forms a bright spot (Ls) on the disk.
The coil (C) arranged on the bottom surface of the supporting base (B) may be formed by printing a selected pattern of a conductive material on the bottom surface of the supporting base (B). As shown in FIG. 20, the coil (C) surrounds the aperture of the through hole (a) that is open in the bottom surface of the supporting base (B). The through hole (a) is arranged to positionally correspond to the nominal optical axis of the objective lens (L).
As shown in FIG. 17, the arm (A) is caused to move radially of the disk (D) by a linear actuator (Ac) such as a voice coil motor. When the apparatus is not operated, the slider (S) is elastically urged to the disk surface by the suspension arm (Sa) to be held in pressed contact with the disk (D). On the other hand, when the disk (D) is caused to rotate at high speed, the slider (S) will float slightly above the disk (D) by a fluid wedge formed between the slider (S) and the disk (D).
As shown in FIG. 17, the conventional apparatus further includes a stationary module (SM) incorporating a laser emitting unit, a detector, a collimator and so on. In operation, a laser beam is emitted from the module (SM) to travel along the arm (A). Then, the laser beam is reflected by the mirror (M) to go along a path perpendicular to the previous path. Consequently, the laser beam enters the objective lens (L) and is converged by the lens. Consequently, a predetermined laser spot (Ls) is formed on the disk (D).
In the conventional apparatus described above, the coil (C) is disposed so adjacent to the disk (D) as to surround the laser spot (Ls). Thus, a magnetic field needed for performing data-recording or data-reading will be properly applied to a region corresponding to the laser spot (Ls) on the disk (D). However, the conventional apparatus has been found disadvantageous in the following point.
As shown in FIG. 18, the coil (C) of the conventional optical head is not provided with a core. With such an arrangement, the magnetic field to be generated by the coil (C) is rendered weaker than the magnetic field to be generated by a coil provided with a core having a high magnetic permeability.
One way to strengthen the magnetic field to be generated by a coil is to increase the number of turns of the coil. However, as the number of the turns of the coil is increased, the resistance of the coil may unfavorably become greater. In such a case, a higher voltage should be supplied to the coil for causing the coil assembly to work properly. Further, a coil with an increased number of turns may fail to provide an expected response as the voltage frequency supplied to the coil becomes higher. Clearly, it is disadvantageous to use such a coil as an external magnetic field generating means for performing recording of data by magnetic field modulation.